1. Field of the Invention
The present invention relates to a continuously variable capacity swash plate type compressor having a plurality of double-headed compressing pistons reciprocating in front and rear cylinder bores of a cylinder block.
2. Description of the Related Art
A typical continuously variable capacity swash plate type compressor is disclosed in Japanese Unexamined (Kokai) Patent Publication No. 1-138382, and is shown in the accompanying FIG. 5. The compressor of FIG. 5 includes an axial cylinder block 51 having a plurality of axially extending cylinder bores 52 therein, a plurality of double-headed pistons 53 reciprocating in the cylinder bores 52, and a drive shaft 54 rotatably supported by the cylinder block 51 in such a manner that the axis of rotation of the drive shaft 54 is in parallel with the cylinder bores 52. The drive shaft 54 is provided with a slide or guide bush 55 slidably mounted thereon. The slide 55 has a spherical supporting portion 54a on which a swash plate 57 is rotatably mounted at a spherical recessed portion 57a thereof in such a manner that a peripheral portion of the swash plate 57 is engaged with the double-headed pistons 53 via shoes 56, respectively. The swash plate 57 is provided with a front connecting portion 57b connected to a front half portion 54a of the drive shaft 54 via a connecting pin 58 movably fitted in guide bores 54b formed in the front half portion 54a. The swash plate 57 is capable of being moved to change an inclination thereof with respect to a plane vertical to the axis of the drive shaft 54 about a center "C" existing at the peripheral portion of the swash plate 57, in response to a sliding movement of the slide 55 on the drive shaft 54, and thus one of the heads of each double-headed piston 53 is able to constantly reach a predetermined position in the corresponding cylinder bore 52, at a top dead center thereof during a compression stroke of the piston 53. Accordingly, even if the compressor is operated at a small capacity condition where the angle of inclination of the swash plate 57 is substantially zero with respect to the plane vertical to the axis of the drive shaft 54, a compression of a refrigerant gas and a discharge of the compressed gas are ensured.
In the conventional compressor of FIG. 5, the angle of inclination of the swash plate 57 is changed and controlled by a pressing force acting on the swash plate 57, due to a total pressure prevailing in the plurality of front and rear side cylinder bores 52, and a counter force acting on the swash plate 57 via a slidable controller 60, due to a pressure prevailing in a control chamber 59, which is connected to either a discharge pressure region of the compressor or a suction pressure region of the compressor and in which the volume thereof is changed by the slidable controller 60 slidable on the drive shaft 54.
The compression of the refrigerant gas is achieved by the reciprocation of the double-headed pistons 53 in the front and rear side cylinder bores 52, due to a wobbling of the inclinable variable swash plate 57 when the drive shaft 54 is rotated. During the compressing motion of the pistons 53, the swash plate 57 is subjected to a moment shown by an arrow M in FIG. 5, due to the pressure generated in the front and rear cylinder bores 52, and accordingly, the connecting pin 58 is pressed against the inner wall of the guide bores 54b to generate an axial force component which forcibly moves the drive shaft 54 forward, i.e., to the left in FIG. 5. Therefore, when the drive shaft 54 is forcibly moved, a reactive force is generated to press the slide 55 rearward, via the swash plate 57, and thus the angle of inclination of the swash plate 57, and accordingly the compressor capacity, is determined to establish a balanced condition between the above-mentioned reactive force and the pressure prevailing in the control chamber 59. Namely, by adjustably changing the pressure in the control chamber 59, the angle of inclination of the swash plate is changed to vary the compressor capacity.
Nevertheless, in the continuously variable capacity swash plate compressor as shown in FIG. 5, when a relationship between the displacement of the controller 60 and the pressure level in the control chamber 59 is graphically designated in an abscissa vs ordinate coordinate system, the relationship is illustrated by a curve as shown in FIG. 6. The curve of FIG. 6 indicates that, in the region of a small compressor capacity, i.e., in the region where an amount of movement of the slidable controller 60 is large, a force acting to press the slide 55 rearward and a counter pressure in the control chamber 59 acting to press the slide 55 forward, which are balanced with one another, are reduced to a very small level, and accordingly, the slide 55 is moved forward, i.e., in a direction which increases the compressor capacity, by an inertial force of the double-headed pistons 53 during a high speed rotation of the drive shaft 54. Namely, the compressor capacity is varied regardless of a change in a cooling load of the compressor or under the influence of a large change in the suction pressure of the refrigerant. Further, since the pressure in the control chamber 59 is maintained at a small pressure level, the slide 55 and the controller 60 cannot be smoothly moved in response to a change in the pressure in the control chamber 59, due to friction, and therefore, the compressor capacity cannot be smoothly varied.
In addition, in the front side cylinder bores 52 of the compressor, when the stroke of the double-headed pistons 53 is decreased, a volume in each front side cylinder bore 52 which is not fully compressed by the piston 53 increases, and accordingly, before the stroke of the pistons 53 is decreased to substantially a zero stroke, a state appears wherein a compression and discharge of the refrigerant is not carried out. More specifically, when the compressor is operated at a capacity of less than 30 through 40% of the full compressor capacity, no compression and discharge of the refrigerant occurs in the front side cylinder bores 52, i.e., the front side cylinder bores 52 remain in an operatively dormant state. When such a dormant state of the front side cylinder bores 52 occurs, a flow of the refrigerant from a swash plate chamber 61 toward the front side suction chamber disappears, and therefore, a thrust bearing 62, a radial bearing 63, and a lip seal 64 arranged on the front side of the compressor are not properly lubricated by the flowing refrigerant gas, and a lubricant oil soluble in the refrigerant gas, during the operation of the compressor at a small capacity.